1. Field of the Invention
The present invention relates to an image display apparatus comprising a light valve.
2. Related Art
In general terms, a light valve is a device which has light transmission or reflection characteristics that can be made to vary with an applied electrical quantity. Liquid crystal display (LCD) panels are one example of a well known device used as a light valve in image display systems within a wide range of equipment. LCD panels are passive (do not generate light) and must be illuminated by an external light source. For instance, conventional LCD panels may either be back lit with a dedicated light source or may be reflective relying on ambient light for illumination. LCD panels have also been developed for use as light valves in image projection systems.
A typical LCD panel comprises a matrix array of liquid crystal cells each of which constitutes a single pixel of the displayed image. The image displayed is determined by the state of each cell which is controlled by appropriate electrical drivers applied to individual columns and lines of cells in the matrix. Typically video data is supplied to column drivers and each line of the panel (and thus the image) is updated in sequence by line drivers. The time duration required to update a single line is referred to as the line period and the time duration between successive updates of any given line is referred to as a frame period. The individual liquid crystal cell settings of each line remain fixed during a frame period so that each line acts as a light valve and the image displayed is flicker free.
A well known problem affecting conventional LCDs is the “smearing” of images, or portions of an image, which move rapidly across the display. For instance, the most widely used form of liquid crystal cell is the twist nematic cell which has a typical response period which falls in the range of 10 to 40 milliseconds. In contrast, video display screens conventionally operate at around 50 Hz thus having a frame period of the order of 20 milliseconds. As the frame period approaches or exceeds the LCD cell response time, visible artefacts, such as loss of contrast of the leading edge and smear of the trailing edge of a moving image element, are introduced into the display with a resultant loss of dynamic resolution.
One known method of combating this problem is to operate the LCD panel at an elevated temperature to reduce the viscosity of the liquid crystal and thereby reduce response time. An improvement of about 30% of the cell response time can be obtained in this way but care has to be taken to ensure temperatures do not exceed the liquid crystal stability limit above which the panel may cease to work.
An alternative approach to combating resolution problems is to apply an over voltage to the liquid crystal of the LCD panel. This approach however improves only the response time between grey levels and has no effect on the smearing from black to white and vice versa.